1. Field of the Invention
The present invention relates to a signal transmission apparatus and method, and, more specifically, it relates to an apparatus and method which transmits a digital signal representative of contents of information as well as converts the digital signal to an analog signal to output, or transmits the analog signal together with the digital signal to determine whether the contents are protected by copyright, and performs copyright protection operation for the analog signal when the contents are determined as protected by copyright.
2. Description of the Prior Art
Conventionally, with respect to a computer device or the like, a digital image signal was converted to an analog image signal and was supplied to a display device by an analog transmission. However, with the widespread use of the display device which drives a display unit with the digital signal, for example a liquid crystal display, a digital transmission method without converting the digital image signal to the analog signal has been standardized to prevent from degradation of image quality and to cut costs or the like. For instance, DVI (Digital Visual Interface, Version 1.0) Standard is worked out by DDWG (Digital Display Working Group). According to the DVI Standard, a digital three primary-color image signal is serially transmitted color-by-color by base-band using a transmission line of TMDS (Transition Minimized Differential Signals) developed by Silicon Image Corporation in U.S.A.
Additionally, with the development of information processing technology, it is being possible to digitize various information of an image or a voice or the like and handle them integrally. For instance, information of a movie, music or the like is digitized and a system of distributing such digitized information among users via communication network is also in practical use.
As such, when various information is digitized and distributed, exact duplicates are easily and inexhaustibly made because information to be distributed is digitized. This provides users with much advantage but presents a problem with respect to protection of legal right on the part of offering works or publications. As a technology for protecting a copyright of digital data, there is the one which is disclosed in Japanese Unexamined Patent Application Publication No. 10-124998, and a technology for controlling output by using terminal information of a terminal unit is disclosed in Japanese Unexamined Patent Application Publication No. 10-63599. Furthermore, regarding the digital image signal to be transmitted by the DVI Standard, a proposal, referred to as DVI-CP (Content Protect)including encryption technology for copyright protection, is made by Intel Corporation in U.S.A.
According to the DVI Standard, not only the TMDS transmission line but also the analog transmission line are established so as to be capable of connecting the display device corresponding to the conventional analog transmission. Therefore, it is necessary that copyright protection should be made for not only the digital image signal transmitted by digital but also the analog image signal.
In addition, the DVI Standard assumes transmission of the image signal between the computer device and the display device. Devices connectable by the DVI Standard, however, are not limited to the computer device and the display device, but a set-top box unit or a television set may be connected. There are many cases where such consumer-electronics products demand a function capable of transmitting the supplied image signal to the other devices. For instance, the television set not only displays the image based on the digital image signal supplied from the set-top box unit, but also has a function of converting the digital image signal to the analog image signal to output so that the displayed image may be recorded in a video tape-recorder. Such being the case, for the apparatus having a function of outputting such analog image signal, copyright protection should also be made for the outputted analog image signal. Otherwise, practical effect of copyright protection may not be exerted.